1. Field of the Invention
The present invention relates to an exposure apparatus which is used to expose a mask pattern on a photosensitive substrate in photolithographic processes used to manufacture (for example) semiconductor elements, liquid crystal display elements, imaging elements (CCD's, etc.) or thin-film magnetic heads. More specifically, the present invention concerns an exposure apparatus that calibrates a permanently installed illumination sensor, which is used for the direct or indirect measurement of the intensity of illumination of the exposing light on the stage on which the photosensitive substrate is placed.
2. Discussion of the Related Art
Conventional exposure apparatus, such as a one-shot exposure apparatus (steppers, etc.) in which a pattern on a reticle (or photo-mask, etc.) is used as a mask, is transferred via a optical projection system onto a wafer (or glass plate, etc.), which is coated with photoresist as the photosensitive substrate. Alternatively, a proximity type exposure apparatus, in which a reticle pattern is transferred directly onto the wafer without using a optical projection system has been used in order to manufacture semiconductor elements, and the like. Recently, scan exposure type projection exposure apparatus, such as step-and-scan systems, are employed in which a reticle pattern is successively transferred onto respective shot areas on a wafer by synchronously scanning the reticle and wafer with respect to the optical projection system. The step-and-scan systems have also been used in order to achieve a substantial increase in the exposure area without increasing the size of the optical projection system.
The amount of exposure that is generally appropriate for the photoresist on the wafer is set. Accordingly, such an exposure apparatus is equipped with an illumination sensor which is used to measure the intensity of the exposing light on the wafer, either directly or indirectly, and the amount of exposure on the wafer is controlled on the basis of the values measured by this illumination sensor.
FIG. 12 depicts a projection exposure apparatus equipped with a conventional illumination sensor. In FIG. 12, exposing light IL emitted by an exposure light source 109 illuminates the pattern area of a reticle R via an illumination system 103, and an image of the pattern on the reticle R is transferred onto each shot area of the wafer W (which is coated with a photoresist) in the presence of the exposing light IL via a optical projection system 102. The wafer W is held on a wafer stage 101, which is used to three dimensional position the wafer.
Furthermore, the illumination system 103 is equipped with a fly-eye lens, a beam splitter 111, which separates a portion of the exposing light emitted from the fly-eye lens, and a condenser lens. The exposing light IL separated by the beam splitter 111 enters an integrator sensor 110, which is used as an illumination sensor (consisting of a photoelectric detector), and the output signal of the integrator sensor 110 is sent to an illumination control unit 105. This illumination control unit 105 is connected to a main controller 107, which controls the operation of the apparatus as a whole, and the main controller 107 is connected to a console 112 which consists of an input device and a display.
The beam splitter 111 has a fixed reflectivity, so that the amount of light which passes through the beam splitter 111 and is directed onto the wafer W can be calculated from the intensity of illumination detected by the integrator sensor 110. In actuality, however, it is necessary to consider this amount of light in accordance with the reflection and absorption occurring in the reticle R and optical projection system 102. Accordingly, as will be described later, the value of the ratio of the output of the integrator sensor 110 to the output of an illumination meter installed on the wafer stage 101 is determined beforehand and stored in the main controller 107. At the time of exposure, the main controller 107 determines the intensity of the exposing light on the wafer W from the value of the above-mentioned ratio and the output value of the integrator sensor 110, and controls the amount of exposing light directed onto the wafer W in accordance with this intensity.
Furthermore, an illumination sensor 104 consisting of a permanently installed photoelectric detector, which is exclusively for use in this apparatus, is fastened to the wafer stage 101 in the vicinity of the wafer W, and the output signal of this illumination sensor 104 is sent to the illumination control unit 105 via a signal cable 106. Then, prior to exposure, the light-receiving part of the illumination sensor 104 is set inside the exposure field of the optical projection system 102, and is moved if necessary so that the intensity of the exposing light on the surface of the wafer stage 101, and any irregularity in this intensity, can be measured.
In this case, since numerous exposure apparatus are installed in a semiconductor elements manufacturing line it is necessary to match the amount of exposure applied to the wafers by the respective exposure apparatus. In order to accomplish such matching, it is necessary to calibrate the outputs of the integrator sensors 110 and illumination sensors 104 installed in respective exposure apparatus with respect to the quantity of incident light (i.e., it is necessary to calibrate the sensitivities of the respective sensors). Specifically, the amount of exposure that is appropriate for the photoresist is determined as follows: wafers exposed under a certain intensity of illumination with the actual exposure time varied are developed, and the appropriate amount of exposure is determined from the exposure time of the image with the highest resolution among the exposed images and the intensity of illumination.
Accordingly, if the sensitivities of the sensors in the respective exposure apparatus are different, the appropriate amount of exposure determined for each exposure apparatus will differ according to these differences in sensitivity. Conventionally, a detachable illumination meter 108, which acts as a reference, has been used to calibrate the sensitivities of the integrator sensors 110 and illumination sensors 104.
In order to allow matching with other exposure apparatus, the illumination meter 108 is constructed so that it is freely detachable from the wafer stage 101 and can be moved over the surface of the wafer stage 101. The output signal of the illumination meter 108 is sent to a display 113 via a signal cable 114, and the value measured by the illumination meter 108 is displayed on the display 113. In this case, the display 113 is installed outside the chamber in which the projection exposure apparatus, shown in FIG. 12, is accommodated so that the display content can easily be seen by the operator. Thus, since the illumination meter 108 is freely detachable from the wafer stage 101, the sensitivities of the integrator sensor 110 and illumination sensor 104 can be calibrated as described below using the illumination meter 108. As a result, when a prescribed amount of exposure (dose) is applied to the photoresist on the wafer, in an arbitrarily selected exposure apparatus, the same amount of exposure can also be applied in the other exposure apparatus.
In cases where sensitivity calibration is performed for the integrator sensor 110. The illumination sensor 104 in the projection exposure apparatus, shown in FIG. 12, the illumination meter 108 is attached in a prescribed position on the wafer stage 101, and this illumination meter 108 is moved to the vicinity of the center of the exposure field of the optical projection system 102, after which the exposure light source 109 is lit by a command from the main controller 107, and the intensity of illumination on the surface of the wafer stage 101 is measured by the illumination meter 108. The output value IA of this illumination meter 108 is displayed by the display 113 and is recorded by the operator.
Next, the permanently installed illumination sensor 104, which is fastened to the surface of the wafer stage 101, is moved to the vicinity of the center of the exposure field of the optical projection system 102, and the intensity of illumination is measured. The output value IB of the illumination sensor 104 is subjected to analog/digital (A/D) conversion in the illumination control unit 105, and the measurement data following this AID conversion is sent to the main controller 107. In parallel with this, the output value IC of the permanently installed integrator sensor 110 is also sampled by the illumination control unit 105, and this sampled output is sent to the main controller 107. The output value IB of the illumination sensor 104 and output value IC of the integrator sensor 110, thus sent to the main controller 107, are sent from the main controller 107 to the console 112, and these output values IB and IC are displayed on the display of the console 112.
From the display the operator determines the value k.sub.AC of the ratio of the output value IA of the illumination meter 108 to the output value IC of the integrator sensor 110 (=IA/IC), with the output of the exposure light source 109 assumed to be constant. The value k.sub.AC of this ratio is a parameter which is used for the calibration of the integrator sensor 110. Specifically, the actual intensity of illumination on the wafer W (corresponding to the intensity of illumination in the other exposure apparatus) can be calculated by multiplying the output value of the integrator sensor 110 during exposure by the parameter k.sub.AC. Furthermore, the value k.sub.AB of the ratio of the output value IA of the illumination meter 108 to the output value IB of the illumination sensor 104 is a parameter which is used for the calibration of the illumination sensor 104. Specifically, the intensity of illumination corresponding to the intensity of illumination in the other exposure apparatus can be calculated by multiplying the output value of the illumination sensor 104 by the parameter k.sub.AB.
The timing with which the output value of the illumination meter 108 (which acts as a reference) is read from the display 113 differs from the timing with which the output values of the illumination sensor 104 and integrator sensor 110 are measured via the console 112. Accordingly, if there is a variation in the output of the exposure light source 109 over time, the parameters k.sub.AC and k.sub.AB measured will fluctuate. Conventionally, the effect of this fluctuation has been alleviated by measuring the output values IA, IB, and IC of the illumination meter 108 over a long period of time and averaging these values, or by calculating the parameters k.sub.AC and k.sub.AB by respectively determining peak hold values obtained in multiple measurements of the output values IA, IB, and IC. In these methods, however, there is no reduction of the discrepancy between the timing of the measurement of the output value IA and the timing of the measurement of the output values IB and IC. Accordingly, the measurement precision of the parameters obtained cannot be greatly increased.
Furthermore, since the display 113, which displays the output value IA of the illumination meter 108 (which acts as a reference) and the console 112, which displays the output values of the illumination sensor 104 and integrator sensor 110 are separated from each other and independently controlled, the measurement results cannot be simultaneously processed. Thus, the calculation of the parameters k.sub.AC and k.sub.AB occur at different points in time.
Moreover, when numerous exposure apparatus are used in a series, there may be instances in which a plurality of illumination meters 108 acting as references are provided, and the permanently installed illumination sensors of the apparatus are calibrated using these illumination meters 108. In such cases, if the matching precision of the outputs of the respective illumination meters 108 is poor, the matching precision of the intensity of illumination in the respective exposure apparatus will decline.